The present invention relates to a process for the manufacture of polycrystalline garnet and to the corresponding monocrystal.
The use of monocrystals having a garnet structure, in particular as substrates for the manufacture of magnetic bubble memory devices, is well known. For this use, the monocrystals of garnet are essentially manufactured by Czochralski drawing of the polycrystalline mixed oxides having the garnet structure, which are hereafter referred to as polycrystalline garnets.
At the present time, polycrystalline garnets are prepared in accordance with three processes.
The first of these processes, described, for example, in Journal of Crystal Growth, 12, pp. 3-8 (1972), consists in following a procedure in which the various oxides forming part of the composition of the garnet are mixed in accordance with the following steps:
the various oxides forming part of the composition of the garnet are calcined separately; PA1 the amounts fixed by the formula of the garnet for the various oxides are weighed; PA1 the oxides are mixed mechanically and the mixture obtained is compacted by compression; PA1 the compacted mixture is introduced into the drawing crucible; and PA1 the mixture is melted.
This process exhibits essentially two disadvantages. On the one hand, during the heating of the mixture, it is impossible to characterize the garnet structure of the polycrystalline mixture, the appearance of which depends on the temperature increase program used; on the other hand, because of the evaporation of certain subspecies, the composition of the bath at the moment of drawing, as a function of its composition at ambient temperature, can only be known from empirical relationships.
The second of these processes (described, for example, in German Pat. No. 2,615,554) consists in following a procedure in which the various oxides forming part of the composition of the garnet are mixed and then calcined so as to obtain the garnet structure prior to the melting operation. The calcination conditions (temperature and time) are determined as a function of the kinetics of solid-solid reactions, which themselves depend essentially on the physical properties of the oxides used and on the proportions of impurities therein.
This second prior art process is the most commonly used process for the preparation of polycrystalline garnets. Compared with the first prior art process, it permits a better knowledge of the composition of the bath at the moment of drawing, because of the possibility of characterizing the garnet structure. However, this process exhibits the disadvantage that it generally requires lengthy calcinations which, as has been demonstrated, do not always result in complete reactions between the oxides. For example, in the case of a Gd.sub.3 Ga.sub.5 O.sub.12 garnet, calcination of a mixture of 3 mols of Gd.sub.2 O.sub.3 to 5 mols of Ga.sub.2 O.sub.3 for 100 hours at 1350.degree. C. results in a mixture in which 10 percent by weight of the Ga.sub.2 O.sub.3 has not reacted.
The third of these processes, described, for example, in Journal of Crystal Growth, 19, pp, 204-208 (1979), consists in thermally decomposing a dissolved mixture of the salts (carbonates, nitrates, chlorides, ammonium salts, sulphates, and the like) of the various elements constituting the garnet, and then subjecting the mixture to a heat treatment until the garnet structure is obtained.
The main disadvantage of this third prior art process is that it leads to impure garnets which are contaminated with the impurities resulting from incomplete decomposition reactions (for example, the presence of rare earth oxychlorides).
Applicants have invented a novel process for the manufacture of polycrystalline garnets of the foregoing types, which make it possible to draw a perfect monocrystal. This process overcomes the disadvantages of the prior art processes and provides numerous advantages. This is an important object of the present invention.
It is also an object of the present invention to provide novel processes for the production of polycrystalline garnets and the corresponding monocrystals.
It is a further object of the present invention to provide novel polycrystalline garnets and their corresponding monocrystals.
Other objects will be apparent to those skilled in the art from the present description, taken in conjunction with the appended drawings.
A related process is disclosed in Applicants' copending U.S. Application Ser. No. 204,894, filed Nov. 7, 1980.